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Author Topic: Grounding stage sections?  (Read 1819 times)

frank kayser

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Re: Grounding stage sections?
« Reply #10 on: March 04, 2014, 03:51:33 pm »

"Ground quality" is a continuum.  Leg caps or boots can frequently be compromised, and stage decking gets banged around all the time.  Aluminum Oxide cannot be depended on in any fashion as an insulator. Bottom line - you have to assume that in the absence of intentional insulators, any metal is conductive and potentially able to be energized.  The larger the metal and/or the larger the contact area with the ground, the larger the current potential.  There's nothing magic about a 6' or 8' ground rod in the ground - a few legs of an aluminum stage with compromised feet on wet grass may have a lower-impedance ground connection than an intentional ground rod in drier soil.

My language may not have been precise enough to fully explain this - an unfortunate reality of this not being billable time.  Two points here that could be expanded on later:

Point 1:
 GFCIs absolutely need a ground connection to function.  This is implemented by the G->N bond in the service entrance, which has the function of completing the circuit of the earth back to the neutral wire.  This is required, otherwise if you touch the hot terminal of your GFCI protected outlet and ground, there is no completed circuit path for the leakage current to go - your body and the ground you're standing on (or the generator, depending on your perspective) would just float at the potential of the hot wire.  This is exactly the isolating scenario of the 120v only Honda Generators I mentioned - a GFCI would never trip, as there is nowhere else for the current to go, since the only possible current sink is the neutral wire. 

Note 1: A GFCI receptacle itself does not necessarily need to be grounded - they are often used to retrofit homes with no ground wire at the receptacle, however the GFCI depends on the environment - the house, stage, etc., being bonded to neutral (done at the service entrance) for there to be a leakage current sink back to the panel.

Note 2: The intent of a GFCI receptacle is to provide a measure of life safety for currents that leak from hot to ground.  A GFCI receptacle provides no safety for a hot -> neutral fault.

Point 2 (Re: in a shore power situation are bonding and grounding the same thing):
From a cord and plug device standpoint, yes.  The green wire in the cord provides the path to earth ground as well as the bonded path back to neutral.  I used the language I did because in the example of staging given, some may think it would be acceptable to drive a ground rod in at the stage and then it would be "grounded".  This is inadequate, as there needs to be a low-impedance path from the conductive stage back to the G/N bond point at the service entrance to sink fault currents sufficiently well to trip the OCPD.  This, functionally, is bonding -  accomplished with a ground wire.


I don't understand your third point question, but absolutely the stage deck should be grounded and bonded in a shore power situation, or any time you have more than a single circuit generator.


Thanks TJ - my apologies for picking apart the language so literally - I do appreciate all I learned in this discussion and that you were spending your personal (non-billable) time.


I also did quite a bit of additional reading in the interim - "No Shock Zone" (Thanks Mike Sokol), and Phil Graham's three-part article in FOH Online.  http://www.fohonline.com/current-issue/74-tech-feature/8858-generators-and-portable-primer-part-1.html
Through your efforts and theirs, I have a much better understanding of grounding vs bonding - the ground rod vs the low impedance path back to the panel needed to trip a breaker and provide safety.


I also have a better handle on the Honda Gen floating neutral/ground issues.  (so I won't be bugging you here).  Still much to read and to sort out.  Thought I had a good handle on it, but it seems I have only scratched the surface.


As for the "ground quality" vis a vis aluminum oxide as an insulator - you're absolutely correct - trust aluminum oxide or a vinyl boot cap as insulators to keep us safe?  Never.  I was looking at these things not as a "protection from ground" but as an interference - high resistance point that could interfere with earth-ground quality.


I guess I had better start drafting a letter to the city manager...
- Speak in broad terms of electrical safety and the likelihood that the stage could become energized
- Refer them to Salina, Kansas
- Get their electrician to produce a plan and get that to the maintenance men who put the stage up.


It's a start...


And again, Thanks TJ!
frank
 
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Mike Sokol

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Re: Grounding stage sections?
« Reply #11 on: March 04, 2014, 04:15:59 pm »

I also did quite a bit of additional reading in the interim - "No Shock Zone" (Thanks Mike Sokol), and Phil Graham's three-part article in FOH Online.  http://www.fohonline.com/current-issue/74-tech-feature/8858-generators-and-portable-primer-part-1.html
Through your efforts and theirs, I have a much better understanding of grounding vs bonding - the ground rod vs the low impedance path back to the panel needed to trip a breaker and provide safety.

I've not had time to jump in on this thread yet, but I have done a lot of thinking about the issue of generator and stage grounding/bonding in general. I've also discussed it at some length with a few generator manufacturers and electrical engineers in an attempt to get my head wrapped around the issues involved. I'll try to come up with a bullet-point list of how to handle stage and generator bonding/grounding, which I'll present here for your critique. Perhaps together we can come up with a basic SOP of how to handle these situations before the outside spring concerts get started. Could save a few lives, and that's a good thing...

Scott Wagner

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Re: Grounding stage sections?
« Reply #12 on: March 04, 2014, 04:33:41 pm »

The city supplies about ten 4x8 sections of staging on approximately 2' legs with vinyl/rubber caps.

The sections do not bolt or clamp together - there is some motion among the sections.
While this reply has nothing to do with electricity, it is important none-the-less.  This condition is unsafe.  Have the city install C-Clamps between the sections.  It's a cheap way to insure that nobody gets injured, and is common practice in staging.
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Scott Wagner
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Stephen Swaffer

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Re: Grounding stage sections?
« Reply #13 on: March 04, 2014, 08:22:58 pm »

Tom.

GFI circuits do not need a ground to function.  They measure the current on the hot and neutral wires and compare them-if they are not within the trip threshhold  of each other-ie current has gone anywhere else than back to the source, be that a gen set or a service-they trip.  GFI receptacles do not need a ground to function, and I have installed enough GFI breakers to know they they have no connection to ground-just to the neutral.  If they did sense a true ground, they could guard against the RPBG that is so dangerous.

OPCDs absolutely MUST have a low impedance ground path to function correctly in all situations.  OPCDs should take care of hot-neutral faults-if sized properly, and if the equipment grounding conductor exists as it should, it will trip on a hot-ground fault, although a GFCI will often react quicker than a  circuit breaker,if there is one in the circuit.
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Steve Swaffer

TJ (Tom) Cornish

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Re: Grounding stage sections?
« Reply #14 on: March 04, 2014, 09:42:02 pm »

Tom.

GFI circuits do not need a ground to function.  They measure the current on the hot and neutral wires and compare them-if they are not within the trip threshhold  of each other-ie current has gone anywhere else than back to the source, be that a gen set or a service-they trip.  GFI receptacles do not need a ground to function, and I have installed enough GFI breakers to know they they have no connection to ground-just to the neutral.  If they did sense a true ground, they could guard against the RPBG that is so dangerous.

You are incorrect - they do need a ground to function - however, it's not the ground at the screw of the GFCI, but rather the bond at the service panel connects the literal ground - the concrete slab, the dirt, etc., to the neutral wire.  Without this, there is no way for any current to leak out and go anywhere - there is no circuit.  Think about it - where would the fault current go?  There's no current unless there's a completed circuit.

This is a common misconception, as you are indeed correct that the ground screw of the GFCI doesn't need to be connected, but that's not the whole story.
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Mike Sokol

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Re: Grounding stage sections?
« Reply #15 on: March 04, 2014, 10:21:39 pm »

You are incorrect - they do need a ground to function - however, it's not the ground at the screw of the GFCI, but rather the bond at the service panel connects the literal ground - the concrete slab, the dirt, etc., to the neutral wire.  Without this, there is no way for any current to leak out and go anywhere - there is no circuit.  Think about it - where would the fault current go?  There's no current unless there's a completed circuit.

This is a common misconception, as you are indeed correct that the ground screw of the GFCI doesn't need to be connected, but that's not the whole story.

First of all, here's a primer I wrote on GFCI theory to get everyone on the same page: http://www.noshockzone.org/rv-electrical-safety-part-viii-gfci/

As you note, a GFCI doesn't need a ground wire connected to the green ground screw to operate. But to be 100% accurate, a GFCI really doesn't need a ground path at all to trip. It's looking for an unbalanced current between the hot and neutral lines. So having some or all the neutral return current go through another neutral path outside of the GFCI's path will unbalance the GFCI sensing circuit and cause it to trip. 

It's all a bit crazy until you draw it out on paper and look at the current paths. But I will agree that anything conductive you can stand on or touch should be bonded/grounded somehow. Just exactly how to do this so it's safe and code compliant under all circumstances will require a bit of thinking. For instance, portable generators...
« Last Edit: March 04, 2014, 10:25:50 pm by Mike Sokol »
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frank kayser

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Re: Grounding stage sections?
« Reply #16 on: March 04, 2014, 11:10:16 pm »

While this reply has nothing to do with electricity, it is important none-the-less.  This condition is unsafe.  Have the city install C-Clamps between the sections.  It's a cheap way to insure that nobody gets injured, and is common practice in staging.

Indeed, if nothing else, the "floating" stage sections are, at best, a real PITA.  At worst, edge sections waiting to snag and cut a cable, energizing the platform.  The floating platform also makes for unsure footing, pinch points that could take a finger, or worse.
Safety is safety - call 'em as you see 'em.  Yeah, another good point to put on my list to talk to the city about.


good call, Scott.
« Last Edit: March 04, 2014, 11:17:49 pm by frank kayser »
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Lyle Williams

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Re: Grounding stage sections?
« Reply #17 on: March 05, 2014, 02:02:22 am »

Grounding the stage creates an opportunity for electrocution.  It is hoped that the opportunity for electrocution pops a breaker or trips an RCD/GFCI before electrocution occurs.  Percentage wise, across lots of incidents, this is absolutely a good thing.  On a case by case basis YMMV.

The stage needs to be built according to the manufacturers specs and local regs.  If there are concerns these should be raised with the installers/manufacturer/engineer/site electrician.
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Stephen Swaffer

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Re: Grounding stage sections?
« Reply #18 on: March 05, 2014, 08:55:08 am »

You are incorrect - they do need a ground to function - however, it's not the ground at the screw of the GFCI, but rather the bond at the service panel connects the literal ground - the concrete slab, the dirt, etc., to the neutral wire.  Without this, there is no way for any current to leak out and go anywhere - there is no circuit.  Think about it - where would the fault current go?  There's no current unless there's a completed circuit.


Technically, you are correct.  However, if there is truly no ground connection and NO current can flow, then there is no hazard , so no current can flow through me, correct?  That is the basic principal of "hot bare hand" work.  (I am NOT advocating doing that kind of work, however unless you are paid and trained to do it!)

If a GFI is in place, with no ground thus no hazard you are safe,  If a ground exists-intentionally or not-you still have the safety net in place.
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Steve Swaffer

TJ (Tom) Cornish

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Re: Grounding stage sections?
« Reply #19 on: March 05, 2014, 09:06:25 am »

Technically, you are correct.  However, if there is truly no ground connection and NO current can flow, then there is no hazard , so no current can flow through me, correct?  That is the basic principal of "hot bare hand" work.  (I am NOT advocating doing that kind of work, however unless you are paid and trained to do it!)

If a GFI is in place, with no ground thus no hazard you are safe,  If a ground exists-intentionally or not-you still have the safety net in place.
My point in these posts has been to draw attention to the fault current path(s) and get people to start thinking about the ground/stage/guitar strings/whatever as a possible current path.  If there is truly no other path - such as the Honda EU2000* which is a completely floating system - then a single fault is not a hazard, and a GFCI would never trip, as there isn't a current path for the fault current to go to.

That said, there is almost always a fault current path, and the mechanism that GFCIs usually trip is due to leakage to ground - bathtub water pipes, wet concrete floor, touching the faucet handle, etc.  This tripping action happens because of the G->N bond at the panel, which is necessary for other parts of the electrical system.

*Disclaimer - I haven't actually measured the impedance between neutral and ground on the EU2000 - there may indeed be a low enough impedance for a GFCI trip to generator frame ground, but I suspect they're just coupling capacitors - I'll try to borrow a generator and measure that sometime.  I do know for sure that the EU6500 has virtually an infinite impedance between neutral and ground.
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